[fitsbits] How to a column to a binary table?

[Tom Kuiper]

I have a binary table extension.  Is there a way to add a column so that 
the table header is properly updated?  Adding the new column to ColDefs 
and calling
new_table(updated_columns,header=old_header)
again, using the old header and columns with the new one added causes a 
second TFIELDS entry. I've tried the
tbhdu.update()
on the bintable HDU after adding the new column to ColDefs but that 
doesn't seem to do it.

Many thanks

Tom